I would have written this 'Marcus magistrum scribentem animadvertit....' Why is 'scribere' in the present infinitive? Surely it's a participle and should be written 'scribentem'?? Or is this some version of infinitive + accusative...but it doesn't make sense to me because that's only used when the action is being reported or commented on by a third person: Marcus 'magistrum scribere' dicit.

The acc. + inf. construction isn't just for speech, but also for various types of perception and mental action. It's found pretty much anywhere where in English you say "he [verb] that", e.g., he saw that..., he heard that..., he imagined that..., he thought that..., he knew that..., he hoped that..., he discovered that..., he threatened that..., he promised that..., etc. It's very common.